So then, what is the precise difference between commercial yeast and a starter? If both came packaged, with microbial ingredients labels on the side, you might not think there was much at all, for both contain many different kinds of yeast and lactic acid bacteria. Yet as in all things relating to microbial leavening, the issue is one of proportion. Whereas in a home-grown starter the bacteria outnumber the yeast by about 100-1 microbe-for-microbe, in a packet of comercial yeast the reverse is true. There, the yeast outnumber the bacteria by about the same ratio.
The upshot of that of course is much faster rising, since there are so many more yeast critters consuming sugars and giving off CO2 and alcohol. The down side however, is there’s less flavor. Or should I say there’s less of a certain kind of flavor, since yeasts produce a heck of a lot of flavor on their own. One of the sources of that flavor is glutamic acid also known as glutamate, which the fermentation process produces in abundance. Glutamate is a molecule known to stimulate the so-called “fifth taste” or umami receptors on the tongue. The flavor it produces can probably be best described as “meaty”. And while nobody will likely ever confuse the flavor of bread with that of meat, it’s a common misconception among bread bakers that yeast is not a flavor-producer. Yeats may not create the “tang” that lactic acid bacteria do, but their contribution to bread’s overall flavor profile is no less important.
So packaged yeast isn’t one-dimensional like most people think. Since it contains the same variety of microbes, it too can be manipulated to create different effects. Making sponges and preferments (like poolishes and bigas) for instance. All are fairly wet mixtures that call for mixing flour, water and packaged yeast, then leaving it to sit for a time…often half an hour to an hour at room temperature, overnight if the sponge is refrigerated. What does that do? The answer is that is allows the lactic acid bacteria that are in with the yeast to catch up a bit and produce some of their own flavor. Granted half an hour isn’t that much (though it can be noticeable), but 8-12 hours can do quite a lot. Many types of lactic acid bacteria can keep working even while they’re in the refrigerator you see, which means they keep producing acid while the yeasts go to sleep (and sleep they must for that long, otherwise they’d consume every last molecule of sugar).
So, lowly packaged yeast has a few surprises in it after all, eh?